Hash Join
Hash Join 不需要任何索引來執行，并且在大多數情況下比當前的塊嵌套循環算法更有效。
下面通過實例代碼給大家介紹Mysql 8.0.18 hash join測試，具體內容如下所示：
CREATE TABLE COLUMNS_hj as select * from information_schema.`COLUMNS`;INSERT INTO COLUMNS SELECT * FROM COLUMNS; -- 最后一次插入25萬行CREATE TABLE COLUMNS_hj2 as select * from information_schema.`COLUMNS`;
explain format=treeSELECT COUNT(c1. PRIVILEGES), SUM(c1.ordinal_position)FROM COLUMNS_hj c1, COLUMNS_hj2 c2WHERE c1.table_name = c2.table_nameAND c1.column_name = c2.column_nameGROUP BY c1.table_name, c1.column_nameORDER BY c1.table_name, c1.column_name;
必須使用format=tree（8.0.16的新特性）才能查看hash join的執行計劃：
-> Sort: <temporary>.TABLE_NAME, <temporary>.COLUMN_NAME -> Table scan on <temporary> -> Aggregate using temporary table -> Inner hash join (c1.`COLUMN_NAME` = c2.`COLUMN_NAME`), (c1.`TABLE_NAME` = c2.`TABLE_NAME`) (cost=134217298.97 rows=13421218) -> Table scan on c1 (cost=1.60 rows=414619) -> Hash -> Table scan on c2 (cost=347.95 rows=3237)
set join_buffer_size=1048576000;SELECT COUNT(c1. PRIVILEGES), SUM(c1.ordinal_position)FROM COLUMNS_hj c1, COLUMNS_hj2 c2WHERE c1.table_name = c2.table_nameAND c1.column_name = c2.column_nameGROUP BY c1.table_name, c1.column_nameORDER BY c1.table_name, c1.column_name;
1.5秒左右。
再來看BNL，先創建索引（分別優化了，再對比效果才公平）。
alter table columns_hj drop index idx_columns_hj;alter table columns_hj2 drop index idx_columns_hj2;create index idx_columns_hj on columns_hj(table_name,column_name);create index idx_columns_hj2 on columns_hj2(table_name,column_name);-> Sort: <temporary>.TABLE_NAME, <temporary>.COLUMN_NAME -> Table scan on <temporary> -> Aggregate using temporary table -> Nested loop inner join (cost=454325.17 rows=412707) -> Filter: ((c2.`TABLE_NAME` is not null) and (c2.`COLUMN_NAME` is not null)) (cost=347.95 rows=3237) -> Table scan on c2 (cost=347.95 rows=3237) -> Index lookup on c1 using idx_COLUMNS_hj (TABLE_NAME=c2.`TABLE_NAME`, COLUMN_NAME=c2.`COLUMN_NAME`) (cost=127.50 rows=127)
大約4.5秒?？梢奾ash join效果還是杠杠的。
不得不吐槽下mysql的優化器提示，貌似HASH_JOIN/NO_HASH_JOIN都不生效。
除了hash_join外，mysql 8.0.3引入的SET_VAR優化器提示還是很好用的，可用來設置語句級參數（oracle支持，mariadb記得也支持了的），如下：
mysql> select /*+ set_var(optimizer_switch='index_merge=off') set_var(join_buffer_size=4M) */ c_id from customer limit 1;
SET_VAR支持的變量列表：
auto_increment_incrementauto_increment_offsetbig_tablesbulk_insert_buffer_sizedefault_tmp_storage_enginediv_precision_incrementend_markers_in_jsoneq_range_index_dive_limitforeign_key_checksgroup_concat_max_leninsert_idinternal_tmp_mem_storage_enginejoin_buffer_sizelock_wait_timeoutmax_error_countmax_execution_timemax_heap_table_sizemax_join_sizemax_length_for_sort_datamax_points_in_geometrymax_seeks_for_keymax_sort_lengthoptimizer_prune_leveloptimizer_search_depth variablesoptimizer_switchrange_alloc_block_sizerange_optimizer_max_mem_sizeread_buffer_sizeread_rnd_buffer_sizesort_buffer_sizesql_auto_is_nullsql_big_selectssql_buffer_resultsql_modesql_safe_updatessql_select_limittimestamptmp_table_sizeupdatable_views_with_limitunique_checkswindowing_use_high_precision
總結
以上所述是小編給大家介紹的Mysql 8.0.18 hash join測試,希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對武林網網站的支持！
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